Why did our brains shrink 3000 years ago?

I went to the original paper (https://www.frontiersin.org/articles/10.3389/fevo.2021.74263...) and this graph of their data: https://www.frontiersin.org/files/Articles/742639/fevo-09-74...

Long story short I've got some concerns with their analysis. To me it looks like the difference between now and the recent past is mostly just increased variance with a lot more brains on the smaller side. (This is more visible on the graph on the left, where the data points start to fan out as one gets later).

3000 years is also quite recent, and if this is a true effect I'd wonder why it would show up the same way in the Old World, in Australia, and in the New World, each of which were genetically isolated from the others for much longer than that.

Lines up fairly well with the introduction and spread of substance agriculture, which is thought to produce less healthy individuals than hunter-gathering. Farmers won because they were more numerous than hunters, not because they were individually stronger.

This seems too fast for evolution, but perhaps not genetic expression.

That said, My first question would be how body size and nutrition were controlled for. They say it was done, but the relation may be nonlinear, and this is a fringe effect.

Perhaps related to the collapse of the bronze age?